System and method of broad band optical end point detection for film change indication

ABSTRACT

A system and method for detecting an endpoint during a chemical mechanical polishing process is disclosed that includes illuminating a first portion of a surface of a wafer with a first broad beam of light. A first reflected spectrum data is received. The first reflected spectrum of data corresponds to a first spectra of light reflected from the first illuminated portion of the surface of the wafer. A second portion of the surface of the wafer with a second broad beam of light. A second reflected spectrum data is received. The second reflected spectrum of data corresponds to a second spectra of light reflected from the second illuminated portion of the surface of the wafer. The first reflected spectrum data is normalized and the second reflected spectrum data is normalized. An endpoint is determined based on a difference between the normalized first spectrum data and the normalized second spectrum data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to endpoint detection in a chemicalmechanical polishing process, and more particularly to endpointdetection using optical interference of a broad reflectance spectrum.

2. Description of the Related Art

In the fabrication of semiconductor devises, typically, the integratedcircuit devices are in the form of multi-level structures. At thesubstrate level, transistor devices having diffusion regions are formed.In subsequent levels, interconnect metallization lines are patterned andelectrically connected to the transistor devices to define the desiredfunctional device. As is well known, patterned conductive layers areinsulated from other conductive layers by dielectric materials, such assilicon dioxide. As more metallization levels and associated dielectriclayers are formed, the need to planarize the dielectric materialincreases. Without planarization, fabrication of additionalmetallization layers becomes substantially more difficult due to thehigher variations in the surface topography. In other applications,metallization line patterns are formed in the dielectric material, andthen metal chemical mechanical polishing (CMP) operations are performedto remove excess metallization.

In the prior art, CMP systems typically implement belt, orbital, orbrush stations in which belts, pads, or brushes are used to scrub, buff,and polish one or both sides of a wafer. Slurry is used to facilitateand enhance the CMP operation. Slurry is most usually introduced onto amoving preparation surface, e.g., belt, pad, brush, and the like, anddistributed over the preparation surface as well as the surface of thesemiconductor wafer being buffed, polished, or otherwise prepared by theCMP process. The distribution is generally accomplished by a combinationof the movement of the preparation surface, the movement of thesemiconductor wafer and the friction created between the semiconductorwafer and the preparation surface.

FIG. 1A shows a cross sectional view of a dielectric layer 102undergoing a fabrication process that is common in constructingdamascene and dual damascene interconnect metallization lines. Thedielectric layer 102 has a diffusion barrier layer 104 deposited overthe etch-patterned surface of the dielectric layer 102. The diffusionbarrier layer, as is well known, is typically titanium nitride (TiN),tantalum (Ta), tantalum nitride (TaN) or a combination of tantalumnitride (TaN) and tantalum (Ta). Once the diffusion barrier layer 104has been deposited to the desired thickness, a copper layer 106 isformed over the diffusion barrier layer in a way that fills the etchedfeatures in the dielectric layer 102. Some excessive diffusion barrierand metallization material is also inevitably deposited over the fieldareas. In order to remove these overburden materials and to define thedesired interconnect metallization lines and associated vias (notshown), a chemical mechanical planarization (CMP) operation isperformed.

As mentioned above, the CMP operation is designed to remove the topmetallization material from over the dielectric layer 102. For instance,as shown in FIG. 1B, the overburden portion of the copper layer 106 andthe diffusion barrier layer 104 have been removed. As is common in CMPoperations, the CMP operation must continue until all of the overburdenmetallization and diffusion barrier material 104 is removed from overthe dielectric layer 102. However, in order to ensure that all thediffusion barrier layer 104 is removed from over the dielectric layer102, there needs to be a way of monitoring the process state and thestate of the wafer surface during its CMP processing. This is commonlyreferred to as endpoint detection. Endpoint detection for copper isperformed because copper cannot be successfully polished using a timedmethod. A timed polish does not work with copper because the removalrate from a CMP process is not stable enough for a timed polish of acopper layer. The removal rate for copper from a CMP process variesgreatly. Hence, monitoring is needed to determine when the endpoint hasbeen reached. In multi-step CMP operations there is a need to ascertainmultiple endpoints: (1) to ensure that Cu is removed from over thediffusion barrier layer; (2) to ensure that the diffusion barrier layeris removed from over the dielectric layer. Thus, endpoint detectiontechniques are used to ensure that all of the desired overburdenmaterial is removed.

Many approaches have been proposed for the endpoint detection in CMP ofmetal. The prior art methods generally can be classified as direct andindirect detection of the physical state of polish. Direct methods usean explicit external signal source or chemical agent to probe the waferstate during the polish. The indirect methods on the other hand monitorthe signal internally generated within the tool due to physical orchemical changes that occur naturally during the polishing process.

Indirect endpoint detection methods include monitoring: the temperatureof the polishing pad/wafer surface, vibration of polishing tool,frictional forces between the pad and the polishing head,electrochemical potential of the slurry, and acoustic emission.Temperature methods exploit the exothermic process reaction as thepolishing slurry reacts selectively with the metal film being polished.U.S. Pat. No. 5,643,050 is an example of this approach. U.S. Pat. Nos.5,643,050 and 5,308,438 disclose friction-based methods in which motorcurrent changes are monitored as different metal layers are polished.

Another endpoint detection method disclosed in European application EP 0739 687 A2 demodulates the acoustic emission resulting from the grindingprocess to yield information on the polishing process. Acoustic emissionmonitoring is generally used to detect the metal endpoint. The methodmonitors the grinding action that takes place during polishing. Amicrophone is positioned at a predetermined distance from the wafer tosense acoustical waves generated when the depth of material removalreaches a certain determinable distance from the interface to therebygenerate output detection signals. All these methods provide a globalmeasure of the polish state and have a strong dependence on processparameter settings and the selection of consumables. However, none ofthe methods except for the friction sensing have achieved somecommercial success in the industry.

Direct endpoint detection methods monitor the wafer surface usingacoustic wave velocity, optical reflectance and interference,impedance/conductance, electrochemical potential change due to theintroduction of specific chemical agents. U.S. Pat. Nos. 5,399,234 and5,271,274 disclose methods of endpoint detection for metal usingacoustic waves. These patents describe an approach to monitor theacoustic wave velocity propagated through the wafer/slurry to detect themetal endpoint. When there is a transition from one metal layer intoanother, the acoustic wave velocity changes and this has been used forthe detection of endpoint. Further, U.S. Pat. No. 6,186,865 discloses amethod of endpoint detection using a sensor to monitor fluid pressurefrom a fluid bearing located under the polishing pad. The sensor is usedto detect a change in the fluid pressure during polishing, whichcorresponds to a change in the shear force when polishing transitionsfrom one material layer to the next. Unfortunately, this method is notrobust to process changes. Further, the endpoint detected is global, andthus the method cannot detect a local endpoint at a specific point onthe wafer surface. Moreover, the method of the U.S Pat. No. 6,186,865patent is restricted to a linear polisher, which requires an airbearing.

There have been many proposals to detect the endpoint using the opticalreflectance from the wafer surface. They can be grouped into twocategories: monitoring the reflected optical signal at a singlewavelength using a laser source or using a broad band light sourcecovering the full visible range of the electromagnetic spectrum. U.S.Pat. No. 5,433,651 discloses an endpoint detection method using a singlewavelength in which an optical signal from a laser source is impinged onthe wafer surface and the reflected signal is monitored for endpointdetection. The change in the reflectivity as the polish transfers fromone metal to another is used to detect the transition.

Broad band methods typically rely on using information in multiplewavelengths of the electromagnetic spectrum. U.S. Pat. No. 6,106,662discloses using a spectrometer to acquire an intensity spectrum ofreflected light in the visible range of the optical spectrum. Two bandsof wavelengths are selected in the spectra that provide good sensitivityto reflectivity change as polish transfers from one metal to another. Adetection signal is then defined by computing the ratio of the averageintensity in the two bands selected. Significant shifts in the detectionsignal indicate the transition from one metal to another.

A common problem with current endpoint detection techniques is that somedegree of over-polishing is required to ensure that all of theconductive material (e.g., metallization material or diffusion barrierlayer 104) is removed from over the dielectric layer 102 to preventinadvertent electrical interconnection between metallization lines. Aside effect of improper endpoint detection or over-polishing is thatdishing 108 occurs over the metallization layer that is desired toremain within the dielectric layer 102. The dishing effect essentiallyremoves more metallization material than desired and leaves a dish-likefeature over the metallization lines. Dishing is known to impact theperformance of the interconnect metallization lines in a negative way,and too much dishing can cause a desired integrated circuit to fail forits intended purpose.

Prior art methods typically can only approximately predict the actualend point but cannot actually detect the actual end point. The prior artdetects when the intensity of a few wavelengths change, such as occurswhen a material becomes translucent (e.g., the material becomessubstantially transparent to some wavelengths but not all wavelengths).When the material becomes translucent, the intensities of somewavelengths change because those wavelengths are being reflected by thelayer below the material currently being removed.

Because the event actually detected by the prior art process is when thelayer being removed (such as a metal layer) becomes translucent ratherthan nonexistent (i.e., fully removed), the prior art process must thenpredict an actual end point (i.e., when all of the desired material isactually fully removed). In one example, the actual event detected, thetranslucent point, occurs when the material is 500 Å thick. Fromprevious processes, the CMP process is known to be removing material ata rate of 3000 Å per minute. Therefore, the actual end point ispredicted by the Formula 1 below:

(translucent material thickness)/(material removal rate)=time delay topredicted end point  Formula 1

In current example: (500 Å)/(3000 Å/minute)=10 seconds

Therefore, the prior art CMP process then continues the CMP removalprocess for an additional 10 seconds after the actual detection eventoccurs. Further, this time delay is calculated based on prior experienceand also assumes a constant removal rate.

In view of the foregoing, there is a need for endpoint detection systemsand methods that improve accuracy in endpoint detection.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing asystem and method of broad band optical end point detection. It shouldbe appreciated that the present invention can be implemented in numerousways, including as a process, an apparatus, a system, computer readablemedia, or a device. Several inventive embodiments of the presentinvention are described below.

A system and method for detecting an endpoint during a chemicalmechanical polishing process is disclosed that includes illuminating afirst portion of a surface of a wafer with a first broad beam of light.A first reflected spectrum data is received. The first reflectedspectrum of data corresponds to a first spectra of light reflected fromthe first illuminated portion of the surface of the wafer. A secondportion of the surface of the wafer is illuminated with a second broadbeam of light. A second reflected spectrum data is received. The secondreflected spectrum of data corresponds to a second spectra of lightreflected from the second illuminated portion of the surface of thewafer. The first reflected spectrum data is normalized and the secondreflected spectrum data is normalized. An endpoint is determined basedon a difference between the normalized first spectrum data and thenormalized second spectrum data.

In one embodiment, the first spectrum data includes an intensity levelcorresponding to each of the wavelengths in the corresponding firstspectra. In one embodiment, the second spectrum data includes anintensity level corresponding to each of the wavelengths in thecorresponding second spectra.

In one embodiment, the wavelengths in the first spectra and the secondspectra can include a range of about 300 nm to about 720 nm.

In one embodiment the first spectra and the second spectra can include arange of about 200 to about 520 individual data points.

In one embodiment, normalizing the first spectrum data includessubstantially removing the process related intensity fluctuations whichare removed by substantially removing the corresponding intensityvalues. In one embodiment, normalizing the second spectrum data includessubstantially removing the process related intensity fluctuations whichare removed by substantially removing the corresponding intensityvalues.

In one embodiment, substantially removing the corresponding intensityvalues can include modifying the intensity values of each one of thewavelengths such that the sum the intensity values of each one of thewavelengths is equal to zero and the sum of the squares of the intensityvalues of each one of the wavelengths is equal to one.

In one embodiment, determining the endpoint based on the differencebetween the normalized first spectrum data and the normalized secondspectrum data can include determining a change in the proportions ofnormalized intensity for at least a portion of the plurality ofwavelengths in the first spectra and the second spectra.

In one embodiment, determining the change in the proportions ofnormalized intensity for at least a portion of the wavelengths in thefirst spectra and the second spectra can include converting thenormalized first spectrum data into a first vector and converting thenormalized second spectrum data into a second vector. A distance betweenthe first vector and the second vector can be calculated. The distancebetween the first vector and the second vector can be compared to athreshold distance and if the distance between the first and secondvectors is greater than or equal to a threshold distance, then a changein the proportions of normalized intensity for at least a portion of theplurality of wavelengths in the first spectra and the second spectra isidentified.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings, andlike reference numerals designate like structural elements.

FIGS. 1A and 1B show a cross sectional view of a dielectric layerundergoing a fabrication process that is common in constructingdamascene and dual damascene interconnect metallization lines.

FIG. 2A shows a CMP system in which a pad is designed to rotate aroundrollers, in accordance with an embodiment of the present invention.

FIG. 2B is a endpoint detection system in accordance with one embodimentof the present invention.

FIG. 3 is a diagram showing a portion of a wafer illuminated by a broadband light source during a CMP process, in accordance with oneembodiment of the present invention.

FIG. 4A is a flowchart diagram that illustrates the method operationsperformed in determining an endpoint for a CMP process in accordancewith one embodiment of the present invention.

FIG. 4B is a flowchart diagram of the method operations 450 incalculating a change in proportions for at least a portion of thewavelengths in the first and second spectra in accordance with oneembodiment of the present invention.

FIG. 5A illustrates one received reflected spectrum of data (i.e., shot)in accordance with one embodiment of the present invention.

FIG. 5B illustrates one normalized reflected spectrum of data (i.e. anormalized shot) in accordance with one embodiment of the presentinvention.

FIG. 5C is a three dimensional graphical illustration of severalnon-normalized shots in accordance with one embodiment of the presentinvention.

FIGS. 6 and 7 are graphs of the data shown in FIG. 5C above, inaccordance with one embodiment of the present invention.

FIGS. 8 and 9 are two-dimensional graphs of the data shown in FIG. 5Cabove that have been enhanced in accordance with one embodiment of thepresent invention.

FIG. 10 is a graphical representation of reflected data that has achange of reflecting coefficient by wavelength with time that has notbeen normalized relative to intensity, in accordance with one embodimentof the present invention.

FIG. 11 is a graphical representation of reflected data that has anintensity normalized reflecting coefficient change in accordance withone embodiment of the present invention.

FIG. 12 is a flowchart diagram of method operations for determining anendpoint in accordance with one embodiment of the present invention.

FIG. 13 is a graph of the vector distance squared (VD) of a materialremoval process in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Several exemplary embodiments for optically determining an endpoint willnow be described. It will be apparent to those skilled in the art thatthe present invention may be practiced without some or all of thespecific details set forth herein.

An important control aspect of the chemical mechanical polishing (CMP)system is determining when the process is at an end, i.e., when to stopthe CMP process. Prior art systems described above, typically predictand endpoint based on various detected data points but cannot accuratelydetect an exact endpoint as will be described in more detail below.

FIG. 2A shows a CMP system in which a pad 250 is designed to rotatearound rollers 251, in accordance with an embodiment of the presentinvention. A platen 254 is positioned under the pad 250 to provide asurface onto which a wafer will be applied using a carrier 252. Endpointdetection is performed using an optical detector 260 in which light isapplied through the platen 254, through the pad 250 and onto the surfaceof the wafer 200 being polished, as shown FIG. 2B. In order toaccomplish optical endpoint detection, a pad slot 250 a is formed intothe pad 250. In some embodiments, the pad 250 may include a number ofpad slots 250 a strategically placed in different locations of the pad250. Typically, the pad slots 250 a are designed small enough tominimize the impact on the polishing operation. In addition to the padslot 250 a, a platen slot 254 a is defined in the platen 254. The platenslot 254 a is designed to allow the broad band optical beam to be passedthrough the platen 254, through the pad 250, and onto the desiredsurface of the wafer 200 during polishing.

By using the optical detector 260, it is possible to ascertain a levelof removal of certain films from the wafer surface. This detectiontechnique is designed to measure the thickness of the film by inspectingthe interference patterns received by the optical detector 260.Additionally, the platen 254 is designed to strategically apply certaindegrees of back pressure to the pad 250 to enable precision removal ofthe layers from the wafer 200.

FIG. 3 is a diagram showing a portion of a wafer 300 illuminated by abroad band light source during a CMP process, in accordance with oneembodiment of the present invention. The wafer 300 includes a siliconsubstrate 302, an oxide layer 304 disposed over the substrate 302, and acopper layer 306 formed over the oxide layer 304. The copper layer 306represents overburdened copper formed during a Damascene CMP process.Generally, the copper layer 306 is deposited over the oxide layer 304,which is etched in an earlier step to form trenches for copperinterconnects. The overburden copper is then removed by polishing toexpose the oxide layer 304, thus leaving only the conductive lineswithin the trenches. Dual Damascene occurs in a similar manner andallows the formation of metal plugs and interconnects at the same time.

During the polishing process, an optical endpoint detection system usesthe optical interference to determine when the copper 306 has beenremoved. Initially, shown in view 301 a, the copper layer 306 isrelatively thick (e.g., about 10,000 Å) and thus opaque. At this point,the light 308 that illuminates the surface of the wafer 300 is reflectedback with little or no interference. As the copper is polished down, thecopper layer 306 becomes a thin metal (e.g., at about 300-400 Åthick).This is known as the thin metal zone. At this point, shown in view 301b, the copper layer 306 becomes transparent to at least some wavelengthsof the light 312 and those wavelengths can pass through the copper layer306 to illuminate the layers beneath.

When some wavelengths of the light 312 begin illuminating the layer 304,other wavelengths of the light 312 continue to reflect back from thesurface of the thin metal zone copper layer 306. The intensity of thereflected wavelengths of light 318 that are reflected from the interfacebetween layer 304 and layer 302, below the copper layer 306, isdifferent than the intensity of the same wavelengths of light 314reflected from the copper layer 306. However, only the intensities ofthe wavelengths that are reflecting from the interface between layer 304and layer 302 will change. The intensities of the remaining wavelengthsof light 314 that are reflected from the copper layer 306 will notchange.

One reason the intensities of the wavelengths of light 318 change is dueto the fact that each of the various layers 302-306 have a correspondingreflective index. The reflective index impacts the intensity of thelight reflected from that layer.

As the copper is fully removed, as shown in view 301 c, the copper layer306 is no longer present to reflect or block the passage of any of thewavelengths of the light 322. Therefore, all wavelengths of the light322 can then illuminate the layer 304 that lies below the copper layer306. Substantially all wavelengths of the light 324 reflected back fromthe layer 304 will have a change in intensity level as compared to theintensity of the same wavelengths of light reflected from the copperlayer 306.

The optical detector 260 detects the reflected light 308, 314, 318, 324.Therefore, in one embodiment of the present invention, an endpoint isdetermined when substantially all of the wavelengths of the reflectedlight experience a change in intensity.

Thus, when the copper layer 306 is thick, the intensities of thewavelengths of light 308 do not change. However, multiple otherinterference source such as slurry thicknesses, belt interference, andother sources can cause intensity “noise” that can cause the intensitiesof all the wavelengths of reflected light to change. Therefore, theendpoint must be differentiated from the various intensity noisesources. In one embodiment, the present invention can detect the actualend point and differentiate that endpoint from various intensity noisesources.

FIG. 4A is a flowchart diagram that illustrates the method operationsperformed in determining an endpoint for a CMP process in accordancewith one embodiment of the present invention. In operation 402 a firstportion of a surface of a wafer is illuminated with first beam of broadband light. In operation 404, a first reflected spectrum data (i.e., afirst shot) is received. The first shot corresponds to a first set ofspectra of light reflected from the first illuminated portion of thesurface of the wafer. In one embodiment, the first shot includes anintensity level corresponding to each of several wavelengths in thecorresponding first spectra. In one embodiment, the first reflectedspectra are in the range of about 200 nm to about 720 nm wavelengths.The number of individual wavelengths that can be detected is limitedonly by the ability of the optical detector 260. In one embodiment, 512individual wavelengths are detected, however, fewer or a greater numberof individual wavelengths can also be detected.

In operation 406 a second portion of a surface of a wafer is illuminatedwith second beam of broad band light. In operation 408, a secondreflected spectrum data is received (i.e., a second shot). The secondshot corresponds to a second set of spectra of light reflected from thesecond illuminated portion of the surface of the wafer.

FIG. 5A illustrates one received reflected spectrum of data (i.e., shot)in accordance with one embodiment of the present invention, such as thefirst shot received in operation 404 of FIG. 4A above. Approximately 512individual wavelengths are shown across the x-axis. The intensity isshown on the y-axis.

Referring again to FIG. 4A, in operations 410 and 412, respectively, thefirst shot and the second shot are normalized. According to oneembodiment, normalizing the first shot and the second shot includessubstantially removing the intensity aspect from the shots. In oneembodiment, the intensity is substantially removed by adjusting theintensity of each of the detected wavelengths such that a sum of thetotal intensities of all detected wavelengths is equal to zero and sumof squares of the total intensities of all detected wavelengths is equalto one.

FIG. 5B illustrates one normalized reflected spectrum of data (i.e. anormalized shot) in accordance with one embodiment of the presentinvention, such as the normalized first shot as determined in operation410 of FIG. 4 above. Approximately 512 individual wavelengths are shownacross the x-axis. The intensity is shown on the y-axis. The sum of theintensities is equal to zero and sum of squares of the total intensitiesof all detected wavelengths is equal to one. The method operations ofnormalizing a shot will be described in more detail below.

Referring again to FIG. 4A, in operation 414, a difference between thenormalized first shot and the normalized second shot is determined andis used to determine an endpoint of the CMP process. In one embodiment,determining a difference between the normalized first shot and thenormalized second shot includes determining a change in the proportionsof intensity for at least a portion of the wavelengths in the first andsecond spectra.

FIG. 4B is a flowchart diagram of the method operations 450 incalculating a change in proportions for at least a portion of thewavelengths in the first and second spectra in accordance with oneembodiment of the present invention. In operation 452, the normalizedfirst spectrum data is converted to a first vector. In operation 454,the normalized second spectrum data is converted to a second vector. Inoperation 456, a distance between the first and second vectors iscalculated. The distance between the first and second vectors iscompared to a threshold distance to determine if the distance betweenthe first and second vectors is greater than or equal to a thresholddistance, in operation 458. If the distance between the first and secondvectors is greater than or equal to the threshold distance, then achange in proportions of the intensity is identified for at least aportion of the wavelengths in the first and second spectra, in operation460 and the method operations end.

FIG. 5C is a three dimensional graphical illustration of severalnon-normalized shots in accordance with one embodiment of the presentinvention. The wavelengths in nm ranging from approximately 200 nm atthe origin end of the z-axis to approximately 800 nm. Intensity is shownon the y-axis. The x-axis shows the number of shots, approximately 13shots (shots 3-15) are shown. The number of shots shown can correspondto CMP processing time (i.e., polishing time). In one embodiment, thesampling rate is a function of the polishing belt speed and the amountof the end point detection windows in the belt. A line in the x-axis isdrawn to connect the intensity of a given wavelength in a first shot tothe intensity of the same wavelength in a subsequent shot. For example,pointer 551 identifies intensity level of approximately 310 nm in shot 3(shots 0-2 are not shown). Pointer 552 identifies the correspondingintensity level of the same 310 nm wavelength in shot 4. The intensitiesof the various detected wavelengths vary from shot to shot but thevariations are substantially proportionate in that the intensities ofall wavelengths shift upward or downward at the same time. Thisindicates noise in the intensity dimension but does not indicate achange in the actual surface material reflecting the shot.

On the 13^(th) shot (pointer 555) begins a marked downward trend in theintensities of all wavelengths, for subsequent shots 14 and 15, isshown. The downward trend indicated by pointer 555 identifies a changein the material reflecting the shot.

FIGS. 6 and 7 are graphs of the data shown in FIG. 5C above, inaccordance with one embodiment of the present invention. In FIG. 6, thereflected data includes unwanted information such as absolute intensitychanges that result in wide variations in the intensity of the reflectedlight for each of the shots shown.

Conversely, FIG. 7 illustrates the same reflected data that has beennormalized to a relative intensity. Normalizing results in a narrowvariation in the intensity of the reflected light for each of the shotsshown.

The resolution of the reflected data can be increased by analyzing thereflecting coefficient change rather than the absolute intensity value.The reflecting coefficient change can be generated by Formula 2 asfollows:

Change={{right arrow over (R)} _(i) −{right arrow over (R)} ₁}  Formula2

A change in the reflecting coefficient can indicate an endpoint (i.e.,when the desired layer is fully removed).

FIGS. 8 and 9 are two-dimensional graphs of the data shown in FIG. 5Cabove that have been enhanced in accordance with one embodiment of thepresent invention. In FIG. 8, the absolute value of the reflectingcoefficient by wavelength and time is shown. In FIG. 9, the change ofreflecting coefficient by wavelength with time is shown. This stepsprovides a characteristic signature of the film (material reflecting thelight) is dependent on an interference effect. The characteristics oftransparent film, i.e., where two surfaces meet, will reflect the light.For copper processes, the change in reflected data includes changingfrom opaque in visible spectra copper to a transparent film layer belowthe copper layer. After the reflected data is obtained in qualitativefashion described above, the data can be processed to build an endpointdetection based on this change.

FIG. 10 is a graphical representation of reflected data that has achange of reflecting coefficient by wavelength with time that has notbeen normalized relative to intensity, in accordance with one embodimentof the present invention. FIG. 11 is a graphical representation ofreflected data that has an intensity normalized reflecting coefficientchange in accordance with one embodiment of the present invention. FIG.11 demonstrates that measured value changes from straight line 1102,1104 with some high frequency oscillations into well-defined sinusoidalinterference related oscillations 1106, 1108, 1110, 1112 and those lineswith transitional states 1114, 1116.

A second characteristic of transparent films and a function of thicknessand refractory index (not shown) can also influence the reflected data.For example, sinusoidal function of different frequencies relates totransition from one film to another.

FIG. 12 is a flowchart diagram of method operations 1200 for determiningan endpoint in accordance with one embodiment of the present invention.In operation 1210, a reflecting coefficient for wafer for a first shotis calculated according to Formula 3 as follows:

R _(i)(λ_(j))=I _(Wi)(λ_(j))/I _(Li)(λ_(j)), j=1, . . . , 512  Formula 3

In operation 1215, the reflecting coefficient is normalized andpresented in relative intensity units according to Formula 4 as follows:${{{Formula}\quad 4}:{R_{i}^{\prime}\left( \lambda_{j} \right)}} = {{{{R_{i}\left( \lambda_{j} \right)}/\sqrt{S}},\quad {where}\quad S} = {{\sum\limits_{j = 1}^{512}\quad {{R_{i}^{2}\left( \lambda_{j} \right)},\quad j}} = {1,\quad \ldots \quad {\quad,}\quad 512}}}$

In operation 1220, a change in the normalized reflecting coefficient(i.e., the change in material) is calculated according to Formula 5 asdescribed above follows:

THE Change={{right arrow over (R)}′ _(i) −{right arrow over (R)}′_(k)}  Formula 5

In operation 1225, the vector distance square (VD) between the currentR′_(i) and a pre-selected recipe reference value R′_(k) is calculatedaccording to Formula 6 as follows:${{{Formula}\quad 6}:{VD}} = {{\sum\limits_{J = 1}^{512}\quad {\left\{ {{{\overset{->}{R}}_{i}^{\prime}\left( \lambda_{j} \right)} - {{\overset{->}{R}}_{k}^{\prime}\left( \lambda_{j} \right)}} \right\}^{2},\quad j}} = {1,\quad \ldots \quad {\quad,}\quad 512}}$

In operation 1230, the calculated vector distance is compared to athreshold vector distance. The threshold VD can be a known change invector distance that was determined from previous experience withremoving the layer to be removed to reveal an underlying layer, in oneembodiment. Alternatively, the threshold VD can be a pre-selected numberindicating a direction in the change (e.g., an upward or a downwardtrend in the normalized reflecting coefficient. If the calculated VD isnot greater than the threshold VD, then the I_(wi)(λ) and the I_(Li)(λ)are input to operation 1210 as described above and the method operations1210-1230 repeat. If, however, in operation 1230, the calculated VD isgreater than or equal to the threshold VD, then the end point has beendetermined and the CMP process can be stopped immediately.

FIG. 13 is a graph of the vector distance squared (VD) of a materialremoval process in accordance with one embodiment of the presentinvention. The y-axis is the VD. The x-axis is time or more preciselyshot number. From the origin to approximately the 12^(th) shot, thegraph shows the VD remains approximately constant value. The VD betweenthe 12^(th) shot and the 13^(th) shot are much greater as shown by thegraph. The change in the VD illustrated at the 12^(th) shot indicatesthe endpoint has been detected.

While various aspects and embodiments of the invention have beendescribed above relating to determining an endpoint when removing acopper layer, it should be understood that the methods and systemsdescribed herein can be similarly applied to the removal process of anyother material. The methods and systems described herein can besimilarly applied to the removal of other opaque or non-opaque materialsthat are overlaying a different opaque or non-opaque material. By way ofexample, methods and systems described herein can be used to determinean endpoint of the removal process for removing an oxide layer(non-opaque layer) over a copper layer (opaque layer). Similarly, anendpoint for removing an oxide layer (non-opaque layer) over anothernon-opaque material layer.

While various aspects and embodiments of the invention have beendescribed above relating to determining an endpoint using 512 separatedata points (e.g., wavelengths) along the spectrum of the reflectedbroad band light (e.g., Formula 6 wherein j=1-512). However, the presentinvention is not limited to only 512 separate data points and any numberof data points can be used. The number of data points used is analogousto the granularity of the data received. For finer resolution of thedata, a greater number of individual data points must be collected andused. However, the greater number of individual data points that arecollected also increases the computational load. 512 individual datapoints are used to illustrate one level of granularity of the process.Fewer individual data points such as about 200 or less can also used.Alternatively, additional wavelengths can also be used such as more thanabout 520 data points.

As discussed herein two different scales are used for the same broadbandwidth light. A first scale is the wavelengths included in thespectrum of the broad band light. In one embodiment the spectrum of thebroad band light is from about 300 to about 720 nm. However, thespectrum of broadband light that is used can be expanded to includeshorter and/or longer wavelengths of light. In one embodiment thespectrum of broadband light is selected to correspond to the materialsbeing processed in the CMP process. In one embodiment, a wider spectrumcan be used for a wider variety of materials.

A second scale used to describe the detection of the broad bandwidthlight is the number of data points that are distributed across thespectrum of the broadband light. In one embodiment, if the number ofdata points is 512 and the broadband spectrum is from about 300 to about720 nm, then the first data point corresponds to a wavelength ofapproximately 298.6 nm and the 512 data point corresponds to wavelengthapproximately 719.3 nm. The number and distribution of the data pointsacross the broadband spectrum is determined by the particularmanufacturer of the optical detector. Typically, the data points areevenly distributed across the spectrum. The data points can also bereferred to as a pixel.

With the above embodiments in mind, it should be understood that theinvention may employ various computer-implemented operations involvingdata stored in computer systems. These operations are those requiringphysical manipulation of physical quantities. Usually, though notnecessarily, these quantities take the form of electrical or magneticsignals capable of being stored, transferred, combined, compared, andotherwise manipulated. Further, the manipulations performed are oftenreferred to in terms, such as producing, identifying, determining, orcomparing.

Any of the operations described herein that form part of the inventionare useful machine operations. The invention also relates to a device oran apparatus for performing these operations. The apparatus may bespecially constructed for the required purposes, or it may be ageneral-purpose computer selectively activated or configured by acomputer program stored in the computer. In particular, variousgeneral-purpose machines may be used with computer programs written inaccordance with the teachings herein, or it may be more convenient toconstruct a more specialized apparatus to perform the requiredoperations.

The invention can also be embodied as computer readable code on acomputer readable medium. The computer readable medium is any datastorage device that can store data that can be thereafter be read by acomputer system. Examples of the computer readable medium include harddrives, network attached storage (NAS), read-only memory, random-accessmemory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer systems so that thecomputer readable code is stored and executed in a distributed fashion.

It will be further appreciated that the instructions represented by theoperations in FIGS. 4A, 4B and 12 are not required to be performed inthe order illustrated, and that all the processing represented by theoperations may not be necessary to practice the invention. Further, theprocesses described in FIGS. 4A, 4B and 12 can also be implemented insoftware stored in any one of or combinations of computer readablemedium.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A method for detecting an endpoint during achemical mechanical polishing process, comprising: illuminating a firstportion of a surface of a wafer with first beam of broad band light;receiving a first reflected spectrum data corresponding to a firstplurality of spectra of light reflected from the first illuminatedportion of the surface of the wafer; illuminating a second portion ofthe surface of the wafer with second beam of broad band light; receivinga second reflected spectrum data corresponding to a second plurality ofspectra of light reflected from the second illuminated portion of thesurface of the wafer; normalizing the first reflected spectrum data;normalizing the second reflected spectrum data; and determining anendpoint based on a difference between the normalized first spectrumdata and the normalized second spectrum data.
 2. The method of claim 1,wherein the first spectrum data includes an intensity levelcorresponding to each of a plurality of wavelengths in the correspondingfirst spectra.
 3. The method of claim 2, wherein the plurality ofwavelengths in the corresponding first spectra includes a range of about300 nm to about 720 nm.
 4. The method of claim 3, wherein the pluralityof wavelengths in the corresponding first spectra includes a range ofabout 200 to about 520 individual data points.
 5. The method of claim 1,wherein normalizing the first spectrum data includes substantiallyremoving the corresponding intensity values.
 6. The method of claim 5,wherein, substantially removing the corresponding intensity valuesincludes modifying the intensity values of each one of the plurality ofwavelengths such that the sum the intensity values of each one of theplurality of wavelengths is equal to zero and the sum of the square ofthe intensity values of each one of the plurality of wavelengths isequal to one.
 7. The method of claim 1, wherein determining the endpointbased on the difference between the normalized first spectrum data andthe normalized second spectrum data includes determining a change in theproportions of intensity for at least a portion of the plurality ofwavelengths in the first spectra and the second spectra.
 8. The methodof claim 7, wherein, determining the change in the proportions ofintensity for at least a portion of the plurality of wavelengths in thefirst spectra and the second spectra includes: converting the normalizedfirst spectrum data into a first vector; converting the normalizedsecond spectrum data into a second vector; calculating a distancebetween the first vector and the second vector; determining if thedistance between the first and second vectors is greater than or equalto a threshold distance; and identifying a change in the proportions ofintensity for at least a portion of the plurality of wavelengths in thefirst spectra and the second spectra, if the distance between the firstand second vectors is greater than or equal to the threshold distance.9. CMP process tool comprising: a polishing pad having a pad slot; aplaten having a platen slot, the platen slot capable of aligning withthe pad slot during particular points of the chemical mechanicalpolishing process; a broad band light source for illuminating a portionof a surface of a wafer through the platen slot and the pad slot for aplurality of shots; an optical detector for receiving reflected spectrumdata corresponding to a plurality of spectrums of light reflected fromthe illuminated portion of the surface of the wafer for each of theplurality of shots; logic for normalizing a first reflected spectrumdata corresponding to a first shot; logic for normalizing a secondreflected spectrum data corresponding to a second shot; and logic fordetermining an endpoint based on a difference between the normalizedfirst spectrum data and the normalized second spectrum data.
 10. Thesystem of claim 9, wherein the logic for determining the endpoint basedon the difference between the normalized first spectrum data and thenormalized second spectrum data includes logic for determining a changein the proportions of intensity for at least a portion of the pluralityof wavelengths in the first spectra and the second spectra.
 11. Thesystem of claim 10, wherein, determining the change in the proportionsof intensity for at least a portion of the plurality of wavelengths inthe first spectra and the second spectra includes: logic for convertingthe normalized first spectrum data into a first vector; logic forconverting the normalized second spectrum data into a second vector;logic for calculating a distance between the first vector and the secondvector; logic for determining if the distance between the first andsecond vectors is greater than or equal to a threshold distance; andlogic for identifying a change in the proportions of intensity for atleast a portion of the plurality of wavelengths in the first spectra andthe second spectra, if the distance between the first and second vectorsis greater than or equal to the threshold distance.
 12. A system ofdetecting an endpoint comprising: a broad band light source forilluminating a portion of a surface of a wafer through a platen slot anda pad slot for a plurality of shots; an optical detector for receivingreflected spectrum data corresponding to a plurality of spectrums oflight reflected from the illuminated portion of the surface of the waferfor each of the plurality of shots; logic for normalizing a firstreflected spectrum data corresponding to a first shot; logic fornormalizing a second reflected spectrum data corresponding to a secondshot; and logic for determining an endpoint based on a differencebetween the normalized first spectrum data and the normalized secondspectrum data.
 13. The system of claim 12, wherein the logic fordetermining the endpoint based on the difference between the normalizedfirst spectrum data and the normalized second spectrum data includeslogic for determining a change in the proportions of intensity for atleast a portion of the plurality of wavelengths in the first spectra andthe second spectra.
 14. The system of claim 13, wherein, determining thechange in the proportions of intensity for at least a portion of theplurality of wavelengths in the first spectra and the second spectraincludes: logic for converting the normalized first spectrum data into afirst vector; logic for converting the normalized second spectrum datainto a second vector; logic for calculating a distance between the firstvector and the second vector; logic for determining if the distancebetween the first and second vectors is greater than or equal to athreshold distance; and logic for identifying a change in theproportions of intensity for at least a portion of the plurality ofwavelengths in the first spectra and the second spectra, if the distancebetween the first and second vectors is greater than or equal to thethreshold distance.
 15. The system of claim 12, wherein the firstspectrum data includes an intensity level corresponding to each of aplurality of wavelengths in the corresponding first spectra.
 16. Thesystem of claim 15, wherein the plurality of wavelengths in thecorresponding first spectra includes a range of about 300 nm to about720 nm.
 17. The system of claim 16, wherein the plurality of wavelengthsin the corresponding first spectra includes a range of about 200 toabout 520 individual data points.
 18. The system of claim 12, whereinthe logic for normalizing the first spectrum data includes logic forsubstantially removing the corresponding intensity values.
 19. Thesystem of claim 18, wherein, the logic for substantially removing thecorresponding intensity values includes logic for modifying theintensity values of each one of the plurality of wavelengths such thatthe sum the intensity values of each one of the plurality of wavelengthsis equal to zero and the sum of the squares of the intensity values ofeach one of the plurality of wavelengths is equal to one.